The related patents cited above provide authentication methods and devices for preventing counterfeits of both security documents and valuable articles and at the same time offer new means for increasing their attractiveness and aesthetics.
In the present application, we present a new technique for synthesizing dynamically evolving superposition shape images where the image formation process results from the relative spatial layouts of the contributing layers of lenslet gratings. The relative spatial layouts of the layers of lens gratings yield superposition shape images that may have a certain visual similarity with the superposition shape images produced by existing layer superposition methods such as 1D-moiré, level-line moiré, phase shift methods, lenticular methods and 2D moiré methods. However, since lenslet gratings can be created at a much higher resolution than printed gratings they offer a higher protection against counterfeits and at the same time they allow to authenticate documents by viewing the superposed lenslet gratings in transparency mode.
Phase Shift Techniques Phase shift effects have been used in the prior art for the authentication of documents. For example, thanks to the phase change effect, it is possible to make visible a hidden pattern image encoded within a document (see background of U.S. Pat. No. 5,396,559 to McGrew, background of U.S. Pat. No. 5,901,484 to Seder, U.S. Pat. No. 5,708,717 to Alasia and U.S. Pat. No. 5,999,280 to Huang). When a revealing layer formed of a grating of transparent lines or of an array of cylindrical lenslets is superposed on such a document, the pre-designed latent image becomes clearly visible. This phase effect has the particularity that the latent image does not move. When moving the revealing layer on top of the base layer, the latent image foreground becomes alternatively dark and bright. Phase sampling techniques comprising screen element density, form, angle position, size and frequency variations are described in U.S. Pat. No. 6,104,812 to Koltai et. al. McCarthy and Swiegers teach in U.S. Pat. No. 7,916,343 that by applying a vertical phase shift on a horizontal line grating according to the darkness of an original image, one creates a modified grating potentially capable of hiding the latent image. The latent image is revealed by superposing the original grating on top of the modified grating. 1D-moiré techniques (mainly U.S. Pat. Nos. 7,751,608 and 7,710,551) 1D-moiré synthesizing methods, also called band moiré image synthesizing methods are characterized by equations that relate a base layer layout comprising base bands made of vertically compressed instances of a 1D moiré image, a revealing layer layout comprising a grating of sampling lines and the 1D moiré layout resulting from the superposition of the base and revealing layers. The 1D moiré image shapes are a geometric transformation of the shapes embedded within each band of the base band grating. This geometric transformation comprises always an enlargement in one dimension, and possibly a rotation, a shearing, a mirroring, and/or a bending transformation. 1D-moiré synthesizing methods enable creating a base band grating and a revealing line grating that yield upon translation or rotation of the sampling position of the revealing line grating on the base band grating a displacement of the 1D moiré image shapes.Shape Level Line Moiré Synthesizing Techniques (Mainly U.S. Pat. No. 7,305,105)
Shape level line moiré s occur in a superposition image when a base layer comprising a line grating locally shifted according to the elevation of a spatially laid out shape elevation profile is superposed with a revealing layer comprising the unshifted sampling line grating. The layer with the locally shifted line grating embeds the shape elevation profile generated from an initial, preferably bilevel motif shape image (e.g. typographic characters, words of text, symbols, logo, ornament). By modifying the relative superposition phase of the sampling revealing layer grating in superposition with the base layer (e.g. by a translation or rotation), one may observe as shape level line moiré successions of level lines of the shape elevation profile evolving dynamically between the initial motif shape boundaries (shape borders) and shape foreground centers, respectively shape background centers, thereby growing and shrinking. The movement of shape level lines across the motif shape creates visually attractive pulsing motif shapes, for example pulsing symbols such as a pulsing heart. Shape level line techniques have also been published in December 2014 in “S. Chosson, R. D. Hersch, Beating Shapes Relying on Moiré Level Lines, ACM Transactions on Graphics (TOG), Vol. 34 No. 1, November 2014, Article No. 9, 1-10.
Lenticular Image Synthesizing Techniques
Lenticular image synthesizing methods are well described in the background sections of U.S. Pat. No. 8,284,452 to Blum, U.S. Pat. No. 7,255,979 to Weiss and Pilosso, U.S. Pat. No. 5,924,870 to Brosh and Gottfried and U.S. Pat. No. 5,519,794 to Sandor and Meyers. A lenticular image consists of an ordered sequence, matched to a lenticular frequency, of a plurality of images broken down into bands or strips, which are viewed through an array of cylindrical lenslets (lenticular lenses). The period of the grating of cylindrical lenslets is equal to the strip width multiplied by the number of the contributing of images.
Let us call the phase-shift techniques, 1D moiré techniques, shape level line moiré techniques and lenticular image synthesizing techniques “one-dimensional line-oriented” layer superposition techniques. Let us call 2D periodic moiré or 2D random moiré synthesizing techniques “two-dimensional” superposition techniques.
2D Moiré Techniques
2D moiré techniques are based on the moiré intensity profile that is generated by the superposition of a specially designed 2D base layer dot-screen and a revealing layer formed of a 2D array of transparent dots or of spherical microlenses (see U.S. Pat. No. 6,249,588 to Amidror and Hersch, filed Aug. 28, 1995). The base layer dot-screen consists of a lattice of tiny dots, and is characterized by three parameters: its repetition frequency, its orientation, and its dot shapes. When the revealing layer is laid on top of the base layer dot-screen, when both of them have been designed in accordance with 2D moiré layout techniques, there appears in the superposition a highly visible repetitive moiré pattern of a predefined intensity profile shape, whose size, location and orientation gradually vary as the superposed layers are rotated or shifted on top of each other. As an example, this repetitive moiré pattern may comprise any predefined letters, digits or other symbols (such as the country emblem, the currency, etc.). The base layer dot-screen may include dots of gradually varying sizes and shapes, and can be incorporated (or dissimulated) within a variable intensity halftoned image such as a portrait, landscape, or decorative motif, which is generally different from the motif generated by the moiré effect in the superposition. Embodiments of 2D moiré techniques include a revealing array of microlenses superposed with base layer images formed of the combination of antireflection and partially reflecting structures (see U.S. Pat. No. 8,027,093, filed Oct. 4, 2010, inventors Commander et al.). They also include as base layer a planar array of image icons and as revealing layer a planar array of image icon focussing elements (see U.S. Pat. No. 7,333,268, filed Nov. 22, 2004, inventors Steenblik et al.).
Random Moiré 2D and 1D Techniques
U.S. Pat. No. 7,058,202 to Amidror teaches that the superposition of two specially designed correlated random or pseudorandom 2D dot-screens yields a single instance of a moiré intensity profile which consists of single instance of the moiré shape whose size, location and orientation gradually vary as the superposed layers are rotated or shifted on top of each other. U.S. Pat. No. 8,351,087 to Amidror and Hersch teaches a compound layer that displays a dynamically moving single moiré shape instance. This compound layer is formed of the superposition of a base layer and a revealing layer with a gap between them. The layer elements are laid out at s-random locations, the s-random locations of the revealing layer elements being derived from the s-random locations of the base layer elements. The base layer element locations and the revealing layer element locations are therefore strongly correlated. The s-random locations are determined by applying pseudo-random perturbations or displacements to a periodic set of locations. When tilting the compound layer, the superposition of said s-random base and revealing layers yields a single moiré shape instance, that dynamically varies in its size or orientation and/or moves along a trajectory determined by the respective layouts of the base and revealing layers. Layouts are available in which the moiré shape moves along a direction substantially perpendicular to the tilting direction. The base layer may form a halftone image by having its elements large in dark areas and thin in bright areas. It is possible to conceive a moiré shape that is buried and hidden within background random noise, so that it is not visible when the compound layer is not tilted, and it only appears and becomes visible upon tilting the compound layer.
Stereoscopic Depth Perception of Moiré
Elements of theory about stereoscopic vision can be found in the paper by E. Hibbard et al., “On the Theory and Application of Stereographics in Scientific Visualization”, published in the book “From object modelling to advanced visual communication” edited by S. Coquillard, W. Strasser and P. Stucki, Springer Verlag (2004), pp 178-196. The paper “The moiré magnifier” by M C Hutley, R Hunt, R F Stevens and P Savander published in “Pure and Applied Optics: Journal of the European Optical Society Part A Vol. 3 No 2, pp 133-142 already points to the possibility that moiré effects can be seen in stereoscopic vision. The paper by J. Huck, “Moiré patterns and the illusion of depth”, published at the Intl. Conf. of the International Society of Arts, Mathematics and Architecture (ISAMA), June 2004 indicates how to compute the position and period of the moiré light intensity profile resulting from two vertical layers of vertical straight line gratings separated by a given gap and illuminated from behind. U.S. Pat. No. 7,333,268 to R. A. Steenblick, M. J. Hurt and G. R. Jordan describes for the case of 2D moiré s when a moiré is in front and when a moiré is in the back of the superposed 2D layers. In the present disclosure, we show how to calculate and synthesize 1D moiré shapes having a desired perceived depth when viewed stereoscopically by a human.
Prior Art Microlens and Lenticular Lens Superposition Methods
U.S. Pat. No. 7,931,305 to Tompkin and Schilling teaches the creation of a transparent window incorporating microlens fields on both sides of the window. The system may behave as an individual macroscopic lens. Depending on parameters such as lens spacing and lens diameter, various optical effects are obtained. Items of information may be obtained by having different regions with different lens spacing parameters. Optically, these different regions become apparent to the viewer. In contrast to the present invention, U.S. Pat. No. 7,931,305 does not allow to conceive predefined superposition images having a predefined dynamic behavior, such as moving moiré shapes, shapes with level lines travelling from their center to their borders and vice-versa or dynamically moving shapes formed of successively visible shape instances.
U.S. Pat. No. 8,705,175 B1 to Lundgen and Sarda, filed Mar. 14, 2013, priority Apr. 11, 2012, teaches a method of producing a two-sided lenticular film that exhibits an illusion of stripes embedded within the film.
Prior Art Supersposition Image Synthesizing Techniques
In the prior art, phase-shift techniques, 1D or 2D moiré techniques, either repetitive or random, shape level line moiré techniques and lenticular image synthesizing techniques assume that the base layer information is printed or patterned into the base layer along longitudinal 1-dimensional structures such as bands or as 2-dimensional array structures and that a revealing layer is made of a line-oriented 1-dimensional array or respectively of a 2-dimensional array sampling the base layer. This sampling revealing layer is made of transparent lines or of cylindrical lenslets (lenticular lenses) for the 1D case or of substantially spherical lenses for the 2D case. In phase shift techniques, the base layer information comprises, at given locations, base layer structures shifted by a fraction of the revealing layer sampling line period. In 1D moiré techniques, the base layer information comprises the base bands, each base band incorporating base band shapes obtained by a linear or non-linear geometric transformation of the desired 1D moiré shapes. In 2D moiré techniques, the base layer information comprises juxtaposed dot areas containing dot shapes obtained by a linear or non-linear geometric transformation of the desired 2D moiré shapes. In shape level line moiré techniques, the base layer information comprises a line grating or a grating of dither bands locally shifted in proportion to the elevation profile at the current position. In lenticular image synthesizing techniques, the base layer information comprises the bands representing sections of the contributing images. Embodiments include the creation of a compound made of the revealing layer on one side and of the base layer on the other side of a substrate having a given thickness. When tilting this compound, the revealing layer sampling elements sample different parts of the base layer bands and the superposition image evolves dynamically, according to the implemented superposition image synthesizing technique.
In the present disclosure, we propose for both one-dimensional line-oriented and for two-dimensional layer superposition techniques, repetitive or random, to replace the base layer printing or patterning presented in the prior art by the placement of one-dimensional light concentrating lenslets (e.g. cylindrical lenslets) in the background areas of the base layer shapes. Base layer lenslets may be created on one side of a substrate by a roll-to roll-process simultaneously with the creation of the revealing layer sampling lenslets on the other side of the substrate, thus avoiding shift and rotational inaccuracies between the base and revealing layers.